Why dosent my bf believe me?

Well My boyfriend and i have been together since August 25, 2009. it was aroud that same month (august of 09))that i wen to a party with my brother his girl and one of our friends &her boyfriend. we wer all dancing together, and my friend goes "dance with my boyfriend for a while" so i did it was not for to long maybe a few seconds, i realzed it was wrong. i stoped and jus danced with her.i never told my bf that i danced with thisguy. buh i came out dancing with this guy in some pictures. and my boyfriend seen them. i told him it was nothing he should worry about nothing hppend and plus it was my friends guy. He dont belive me. he told me why was i hiding it from him, so now everytime we argue its aabout that. He thinks im messing around wit other guys &that i have other dudes wen i dont, i stoped talking to all my guy friends for him. We been together for 7months and a few weeks. what can i do):